A direct current (DC) circuit breaker device used in a DC power system is significantly different in configuration and operation from an alternating current (AC) circuit breaker device used in an AC power system. A mechanical AC circuit breaker commonly used in an AC power system, such as a gas-blast circuit breaker, a vacuum circuit breaker, and an air-blast circuit breaker, cannot interrupt a current unless a current value becomes zero. Thus, a mechanical AC circuit breaker interrupts a current at the timing of a current value of a fault current becoming zero, which happens for every half cycle of an alternating current.
A mechanical DC circuit breaker device, on the other hand, needs to be designed such that a current value is forced to be zero, since a direct current does not naturally reach a zero point. In addition, depending on the operation of a DC power system, a direction of a direct current flow may be switched to the opposite direction. Thus, a DC circuit breaker device usually needs to be adapted to a bidirectional current.
For example, a DC circuit breaker device illustrated in FIG. 2 of Japanese Patent Laying-Open No. 59-128714 (PTD 1) is known as an example of mechanical DC circuit breaker devices adapted to a bidirectional current and designed to force a current to be zero. The DC circuit breaker device of this document includes two mechanical circuit breakers connected in series with each other, and two backward current generation circuits connected in parallel with the two mechanical circuit breakers and connected in series with each other. Each backward current generation circuit has a capacitor and a reactor connected in series with each other. An injection switch is connected between a node between the two mechanical circuit breakers and a node between the two backward current generation circuits. Upon occurrence of a fault, the injection switch is turned on to zero a current in one of the mechanical circuit breakers through which a current flows in a direction opposite to a direction of a fault current, thereby interrupting the current.
Unlike the mechanical DC circuit breaker device as described above, a DC circuit breaker device including a semiconductor switch does not need to be designed such that a current value is forced to be zero, and can interrupt a current by opening the semiconductor switch. With a semiconductor switch, however, there is an issue of power loss in a normal current-carrying state, that is, in a closed state. This is because, unlike a current flow through metallic contacts as in a mechanical circuit breaker, passing a load current through a semiconductor switch results in Joule heat generation by a resistance component of the semiconductor switch. Since a semiconductor switch usually has the function of carrying a current in one direction, two semiconductor switches are usually connected in series in opposite directions from each other so as to allow for a bidirectional current flow.
In order to avoid the issue of power loss in a semiconductor switch, there is known a DC circuit breaker device including a mechanical circuit breaker provided in parallel with a semiconductor switch. For example, in a current-limiting device described in Japanese Patent Laying-Open No. 10-126961 (PTD 2), a current flows through a mechanical circuit breaker in a normal state, and upon occurrence of a fault, the current is interrupted by the mechanical circuit breaker and commutated to a semiconductor switch, and ultimately, a direct current is limited by the semiconductor switch. By using a very large resistive element such as a lightning arrester as a current-limiting element, the effect is that the current can be practically interrupted.